Root exudates composition

A. Chaboud and M. Rougier, Effect of root density in incubation medium on root exudate composition of axenic maize seedlings, J. Plant Physiol. 131 602 (1991). 

The magnitude and composition of root exudates are a reflection of the physiological condition of the plant imposed by factors related to light (intensity, duration, and quality), temperature, soil pH, anaerobiosis, soil moisture, soil type, and nutritional status. The in-... 

Incubated under sterile (chloroform fumigation) and nonsterile conditions (organic acid application according to the composition of root exudates). 

Phosphorus (P) is one of the major limiting factors for plant growth in many soils. Plant availability of inorganic phosphorus (Pi) can be limited by formation of sparingly soluble Ca phosphates, particularly in alkaline and calcareous soils by adsorption to Fe- and Al-oxide surfaces in acid soils and by formation of Fe/ Al-P complexes with humic acids (94). Phosphorus deficiency can significantly alter the composition of root exudates in a way that is, at least in some plant species, related to an increased ability for mobilization of sparingly soluble P sources (29,31,71). 

It has been found that many environmental factors influence the amount and composition of root exudates and hence the activity of rhizosphere microbial populations. Microbial composition and species richness at the soil-plant interface are related either directly or indirectly to root exudates and thus vary according to the same environmental factors that influence exudation. In es.sence, the rhizosphere can be regarded as the interaction between soil, plants and microorganisms. Figure 2 shows some of the factors associated with these interactions, which will be discussed during the course of the chapter. Here we mention briefly the influence of some plant and microbial factors on root exudation and rhizosphere microbial populations, while soil factors are discussed later. 

Several studies have indicated that the species diversity of indigenous soil communities will influence the species composition of ectorhizosphere populations (176). On mature roots, seasonal successions may be observed as the soil microbial activity varies with temperature, water content, nutrition, and root exudation. Acero et al. (177) found that the composition of alder (Almis) rhizosphere populations alternated between one dominated by Bacillus spp. in autumn and winter and one dominated by Pseudomonas spp. in spring and summer. 

V. Vancura and A. Hanzlikova, Root exudates of plants IV. Differences in chemi-eal composition of. seed and seedlings exudates. Plant and Soil 36 27 (1972). 

S. M. Schwab, R. T. Leonard, and J. A. Menge, Quantitative and qualitative composition of root exudates of mycorrhizal and non-mycorrhizal plant species. Canadian Journal of Botany 62 1227 (1984). 

Table 1 Effect of Iron Stress on Chemical Composition of Root Exudates Collected from Barley Plants, as Determined by Combined NMR and GC-MS Analysis (Fan et al., 1997)...

Root exudation and microbial action produce organic compounds with a range of composition and molecular weights. These compounds interact with the mineral particles, which also vary in size, shape, ciystallinity, and electric charge (Emerson et al. 1986). Interactions between soil mineral particles, organic matter and microbes can occur at many different size scales, because these materials have a large size range in soils (Fig. 7). 

Fig. 2.90. Electropherograms of sterile root exudate before (a) and after incubation with Mesorhizobium loti (b) and Rhizobium leguminosarum bv. trifolii (c). From their UV spectra, peaks 1-6 were identified as resorcinol, rhamnetin, catechin, quercetin glycoside, quercetin aglycone and hesperidin. Differences in the retention times of the same compounds in different samples could be due to the variations in buffer temperature or sample composition. Reprinted with permission from H. L. Steele et al. [213].

BroeckUng CD et al (2008) Root exudates regulate soU fungal community composition and diversty. App Environ Microbiol 74 738... 

Example 1 Citrus oil or citrus root products can Indeed kill slugs if they are applied as a contact agent or act as a slug repellent. Although some patents have been issued on the use of citrus oil as a slug control agent, such control with a contact agent is difficult and the chemical compositions of citrus oil and root exudate are ill-defined. Other obstacles for commercialization of citrus oil are (1) market size is too small to justify the development cost, and (2) an effective delivery method has yet to be developed. 

In summary, the Py-FI mass spectrum shows a great diversity in the molecular rhizodeposit composition which could not be explained by previous chromatographic analyses of root exudates (e.g., Gransee and Wittenmayer, 2000). These focused mainly at the identification and quantification of a priori expected compounds (Fan et al., 2001). Therefore, Py-FIMS may contribute to the detection of previously unknown rhizodeposits and high-molecular-weight products of rhizodeposit interaction with genuine SOM compounds. 

DIBOA was also found in aqueous extracts of rye pollen [64] and it was identified as a component of root exudates from rye (0.7 - 25 pmol / kg fresh weight). However, DIBOA contents in rye exudates differ considerably depending on cultivars (cv. Forrajero Blaer 25 pmol / kg fr wt cv. Tetra Blaer 0.07 pmol / kg fr. wt.). Interestingly, wheat cultivars tested did exude neither DIBOA nor DIMBOA, although roots of the wheat cultivar Alifen contained similar amounts of DIBOA than present in roots of rye cultivars [175]. On the other hand, benzoxazinones have not only been isolated isolated from rye but also from wheat and com exudates [68,176]. Possibly, the developmental stage and plant fitness regulate the composition of root exudate constituents. 

As said above, plant root chemistry may also influence deeply alpine soil microorganism s biomass. It turns out that the particular chemical composition of exudates is a strong selective force in favour of bacteria that can catabolize particular compounds. Plants support heterotrophic microorganisms by way of rhizodeposition of root exudates and litter from dead tissue that include phenolic acids, flavonoids, terpenoids, carbohydrates, hydroxamic acids, aminoacids, denatured protein from dying root cells, CO2, and ethylene (Wardle, 1992). In certain plants, as much as 20-30% of fixed carbon may be lost as rhizodeposition (Lynch and Whipps, 1990). Most of these compounds enter the soil nutrient cycle by way of the soil microbiota, giving rise to competition between the myriad species living there, from microarthropods and nematodes to mycorrhiza and bacteria, for these resources (e.g. Hoover and Crossley, 1995). There is evidence that root phenolic exudates are metabolized preferentially by some soil microbes, while the same compounds are toxic to others. Phenolic acids usually occur in small concentration in soil chiefly because of soil metabolism while adsorption in clay and other soil particles plays a minor role (Bliun et al., 1999). However, their phytotoxicity is compounded by synergism between particular mixtures (Blum, 1996). 

Variations in the composition of root exudates may be one cause of difference in metal uptake between plant species and genotypes (Mench and Martin, 1991). It is important to investigate if the differences in accumulation capacities for metals by willow clones can be related to differences in rhizosphere processes. It is necessary to ask, therefore, how soil chemistry is influenced by the presence of willow roots, if soil chemistry varies between willow clones with properties of low and high metal accumulation and if the mechanisms are metal - or concentration - specific. 

The differences between clones may depend on a combined effect of plant exudate and microbial effects on the exudate (Marschner, 1995). In studies under nonsterile conditions, rhizosphere microbes may alter the chemical composition of root exudates. Therefore, the differences between high and low metal soil condition as well as different metals in spiked soil can be due to toxic metal effects or effects resulting from an excess of chloride on microbes. When comparing various clones the differences in exudate composition could have been due to various microbe-clone relationships. One should, however, keep in mind that a microbe-plant relationship is present in real environment where we also find these metal-accumulation differences between clones. Whether the differences in rhizosphere processes are due to plants alone or a combination with microbial interactions has to be further investigated. 

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